The invention relates to a process for hydrotreating a heavy oil and a light oil by alternately contacting the heavy oil and the light oil in the presence of hydrogen with a catalyst which has hydrogenating properties.
The hydrotreating processes for heavy oils include hydrodesulphurization, hydrodemetallization, hydrocracking and the like. The hydrotreating is generally carried out at elevated temperatures and hydrogen pressures with a catalyst which has hydrogenating properties. Typically the activity of the catalyst declines during the process due to the deposition of coke onto the surface of the catalyst. When the activity of the catalyst has reached an intolerable low level, the catalyst may be regenerated by burning off the coke, e.g., with the aid of an oxygen-containing gas. However, such a regeneration is disruptive of operations and is time-consuming; accordingly, the frequency of regeneration should be reduced as much as possible or it should be avoided all together. The rate of deposition of coke, and accordingly the rate of deactivation of the catalyst, can be reduced by using high hydrogen pressures, e.g., hydrogen partial pressures of 100 kg/cm.sup.2 and higher. However, for many applications the use of the high pressures is unattractive in view of the attendant necessity to employ costly high pressure-resistant apparatus.
It is possible to regenerate a catalyst which is at least partly deactivated after being contacted with a heavy oil by contacting the deactivated catalyst with a light oil in the presence of hydrogen at conditions similar to those used during the hydrotreatment of the heavy oil. During the hydrotreatment of the light oil at least part of the coke deposited on the catalyst is removed. So, it is possible to regenerate a catalyst by replacing the heavy oil feed to the reactor by a light oil feed. After the coke has been removed from the catalyst to a sufficient extent, the light oil feed is replaced again by the heavy oil feed. Such a procedure may be used at relatively low hydrogen pressures. However, a drawback is that contamination of the heavy oil with the light oil will occur, typically requiring additional separation operations.
The invention provides a process which overcomes this drawback by hydrotreating the heavy oil and the light oil in separate reactors.